Method of manufacturing fiberboard

ABSTRACT

A relatively dry, oxygen-poor, heated fluid is provided for conveying wood fibers through the drying cycle subsequent to crushing the wood chips and for also conveying and distributing the wood fibers from the drying cycle onto a mat-forming wire. The primary source of the fluid is the combustion gases from a burner. Secondary fluids including ambient air, recirculated gases from the drying cycle and inert gases may be introduced to cool the fluid to the desired temperature and provide the fluid with the correct amounts of oxygen and water vapor. In reducing the temperature of the fluid, heat may be extracted by suitable heat exchanges and utilized for auxiliary purposes. The gases used in the process are maintained below 350* C., the oxygen content of the gases is maintained below 17 percent and the water vapor content of the gases is maintained below 100 grams per kilogram of dry gases.

[54] METHOD OF MANUFACTURING FIBERIOARD 3 Claims, 8 Drawing Ilgs.

[52] US. Cl. 241/18, 264/37, 264/121, 302/22, 302/28 [51] let. Cl. .302: 21/00, B29j 5/00 [50] Field otSearch 264/122, 121, 37; 302/26, 28,11, 22;241/18, 28,31

[56] References Cited UNITED STATES PATENTS 1,277,333 8/1918 Meakin 302/26 2,757,115 7/1956 Heritage 264/121 2,989,348 6/1961 Reed 302/28 3,371,137 2/1968 Asplund 264/121 United States Patent [1113,630,456

[72] m" M g OTHER REFERENCES 5 C Lafayette, 69 Lyon, M Dust Explosions" by David J. Price, Chemical and Metal- [211 App]. No. 1% lurgica] Engineering, Vol. 24, No. 11, March 16, 1921, pages Filed 4, 5 and 6. [45] l' l 1971 Primary Examinerllobert F. White [32] Priority May 22, 1968 [33] Fm Assistant Examiner-J. R. Hall I [3 1] 5M Anomey-Sughrue, Rothwell, Mlon, Zlnn & Macpeak ABSTRACT: A relatively dry, oxygen-poor, heated fluid is provided for conveying wood fibers through the drying cycle subsequent to crushing the wood chips and for also conveying and distributing the wood fibers from the drying cycle onto a mat-forming wire. The primary source of the fluid is the combustion gases from a burner. Secondary fluids including ambient air, recirculated gases from the drying cycle and inert gases may be introduced to cool the fluid to the desired temperature and provide the fluid with the correct amounts of oxygen and water vapor. ln reducing the temperature of the fluid, heat may be extracted by suitable heat exchanges and utilized for auxiliary purposes. The gases used in the proces are maintained below 350 C., the oxygen content of the gases is maintained below 17 percent and the water vapor content of the gases is maintained below 100 grams per kilogram of dry gases.

Patented Dec. 28, 1971 2 Sheets-Sheet 1 Patented Dec. 28, 1971 2 SheetsShe et z mum METHOD OF MANUFACTURING FIBERBOARD It is known that fiber boards, especially wood-fiber boards, are made by pneumatically spreading the fibers, impregnated with a polymerizable resin, on a suitable support and then compressing them under heat. Boards made by this so-called dry" method are of a far better quality than those made by moist operating processes.

However, installations needed for this type of manufacture involve a considerable hazard, because fibers suspended in atmospheric air are inflammable. The equipment in question must in fact consist of crushers to produce the fibers, a drier,

mostly of the pneumatic type, where the fibers are circulated by hot air, a pneumatic conveyor to carry the dried fibers to a scattering device; the latter spreads them on a forming wire so as to make up the mattress which is ultimately compressed by heating plates.

Experience shows that accidents occurring in such installations derive mostly from ignited particles escaping from the drier or, more frequently, directly from the disk crusher. These ignited particles might be carried quite far and produce flash flames or even explosions.

The source of heat most generally used for drying consists of a chamber containing burners of liquid or gas fuel. The combustion gases are diluted with atmospheric air so as to have a temperature of 250 C. at the inlet of the drier; but even at this temperature it can happen that particles ignite.

Several methods have been tried to remedy these accident risks.

To begin with the heating was done by heat exchange to prevent that ignited particles from the burners get into the drier. However, this solution eliminates only one of the causes, since it does not affect the particles escaping from the crusher. It has the disadvantage of increasing investment costs and reducing output as the fumes discharged by the heat exchanger still contain a considerable quantity of heat which is lost or at least very difficult to recover.

Further, safety devices have been installed to inject a suitable extinguishing fluid into the pneumatic circulating system of drying, conveying, and scattering the fibers; these devices are controlled by supersensitive detectors reacting to temperature increases or sudden increases of pressure.

All such apparatus are complicated and their effectiveness is doubtful.

According to the invention, every hazard of inflammability or explosion is radically avoided in an installation for making fiber boards by the dry operating process, when these operations of crushing, drying, pneumatic conveyance and scattering the fibers on the forming wire are carried out in an oxygenpoor atmosphere of, for instance, less than 17 percent of said gas.

In fact, it is well known that below this amount of 17 percent fire cannot spread in a suspension of dust or combustible dry wood fibers, irrespective of the ratio solid/gas.

The system proposed by the invention is economical, because is does not require an exchanger between the burner chamber and the drier. It permits the recovery of calories by air condenser, exchange chamber of water vaporization, etc... Moreover, the system proposed by the invention has the further advantage to eliminate the risk of drying temperatures above 250 C. Raising the initial drying temperature to 280 C., even to 350C., permits to reduce the size of the drier and, consequently, to economize on installation costs.

By a further characteristic of the invention, the low oxygen content of the combustion gases discharged by the chamber of burners and providing the hot and dry gases required for drying the fibers is achieved by adding to them not only atmospheric cooling air, but also recirculated gases discharged by the drier.

According to a first variant of the invention, the gases discharged by the drier or entering it are not circulated in the piping system which assures the pneumatic conveyance and the scattering of the fibers on the forming wire.

According to another variant, a second, substantially closed circulation is thus provided for the pneumatic conveyor, the

apparatus for scattering and distributing the fibers on the forming wire and the suction or exhaust chests, associated with the forming wire box. As on the other hand, this second circulation cannot be entirely gastight, it will be supplied either continually or discontinually with a certain supplementary amount of hot gas from the drier circulation. As a variant, this supplementary gas can be derived directly from the burner chamber by means of interposing a cooling device of relatively low performance, since it is intended only as a supplementary output, of which, moreover, the calories can be recuperated. By a further variant, this additional gas supply for the second circulating system can be provided by a source of inert gas, for example, nitrogen or carbonic gas.

It is of course necessary to restrict leakages from this second pipe system as far as possible. As a device especially proposed by the invention, a roller, pressing on the mattress made in the forming chamber or box on the wire gauze, is placed for this purpose at the exit of the box, and other airtightening appliances between his roller and the walls surrounding it.

In any case, whether it concerns the first or the second circulating system, it is of advantage to provide an auxiliary source of inert gas operated by a detector sensible to the oxygen proportion contained in the air of the respective circulation, so that, if for any reason whatsoever, the oxygen percentage would rise above the limit considered hazardous, the source in the question would send its inert gas into the pipes and radically eliminate every risk of ignition.

The attached drawing, given by way of an example, will permit a better understanding of the invention, its special features, and the advantages it offers.

FIG. 1 is a diagrammatic representation of that part of an installation for making wood-fiber boards which concerns the preparation of the fibers, showing the principles of the invention applied to it.

FIGS. 2 and 3 illustrate two variants regarding the cooling of the gas discharged by the drier and the use made of the waste heat thus recovered.

FIG. 4 is a diagram of that part of the equipment which concerns the forming of the fiber mattress.

FIG 5 is a detail section of the distribution chamber and the suction box of that part of the equipment.

FIG. 6 is a section along VI-VI.

FIG. 7 is a detail section showing one end of the axis of the compressing roller.

FIG. 8 is an end view of this axis.

FIG. 1 represents a crusher l, for example, a disk crusher, into which wood chips are introduced through an appropriate opening, taking care, however, to avoid communication with the surrounding atmosphere. This crusher is connected by a pneumatic conveyance pipe 2 with a drier 3, in turn is connected by pipe 4 with a separator 5. The latter has an air-lock 5a to isolate it from the surrounding air, a lower outlet 6 for dry fibers, and an upper outlet 7 for air practically free of the fibers it had to convey. Pipe 7 leads to a suction fan 8, having an outlet 9 linked up with two pipes: a first one 10, leading a suitable outlet, and a second one 11, intended to assure the recirculation, as will be described hereunder. Shutters l2 and I3, suitably coupled with one another, permit the adjustment of the relative proportions of drained and recirculated gas.

To provide the pneumatic equipment with hot, dry, and oxygen-poor gas, there is a combustion chamber 14, enclosing burners l5 and burning, for example, a liquid fuel. Near the outlet of this combustion chamber, there is an air inlet 16, adjustable by shutter 17. The mixture of combustion gas and atmospheric air discharged by chamber 14 enters pipe l8, into which opens recirculating pipe 11. Further on this pipe 18 separates into pipe I9, leading into crusher l, and pipe 20, which communicates with outlet pipe 2 of the crusher, in this manner fonning a bypass; pipe 20 is regulated by shutter 21.

In practice burners l5 and air entries 15a associated with them are regulated in such a way that they work within chamber 14 with an air excess of approximately 33 percent. Under these conditions, and if burners l5 burn a heavy fuel oil of average quality, the combustion gases will have a temperature of 800 C., with an approximate percentage of oxygen amountingto 14.3 percent. From pipe 16 a sufficient quantity of the surrounding air is added to reduce the temperature to approx. 560 C., which corresponds to an oxygen content of about 16.5 percent, i.e., less than the limit value of 17 percent, considered to involve a risk of inflammability or explosion.

This first or primary mixture contains about 16 g. of water vapor per kilogram of dry gas.

To this primary mixture a sufficient quantity of gas is added from recirculating pipe 11, to reduce the temperature to 350 C. in the secondary mixture thus obtained. Granting that the recirculated gas has a temperature of approximately 100 C. and contains about 183 g. of water per kg. of dry gas, this secondary mixture will have at 350 C. roughly 83 g. of water vapor per kilogram. It is easy to see that under these circumstances the mixture still possesses a capacity of water absorption amounting to about 100 g. per kg. of dry gas; this represents a very suitable drying potential and assures an excellent calorific efficiency with a reduced size of drier 3.

The temperature of the secondary mixture, arriving in this state into drier 3, will be reduced in it and in the rest of the circulation to return to 100 C. at the point where recirculating pipe joins outlet pipe 18 at chamber 14.

As to the oxygen content, it is easy to realize that it remains a constant 16.5 percent, since the recirculated gas discharged by pipe 11 is of the same composition as the primary mixture discharged by chamber 14, except for its water vapor content.

It is a matter of course that the temperature and humidity values given here are only indicatory and can vary. If for instance, a temperature of 280 C. is considered for the secondary mixture circulating in pipe 18, it is not difficult to calculate that the water absorption capacity of this mixture will be 76 g. per kg. The functioning is assured but, evidently, with a larger drier for an evaporation of the same effect.

At all events,.the oxygen content of the secondary mixture is inferior to the limit amount considered hazardous, so that there is no longer a possibility for particles to ignite, whereby the initial cause of flash flames and explosions is eliminated. One can see, more particularly, that the crusher is traversed, at least in part, by a gas mixture originating in pipe 18, with the effect that, even if local phenomena of overheating of any importance 9 would occur in the crusher, causing the thermic decomposition of fiber particles, they cannot provoke combustion in the proper sense. I

As a variant, crushers working under steam pressure could, of course, be utilized, which would automatically eliminate the risk of ignitions in the crusher. But evidently, in this case it would not be possible to circulate the secondary gas mixture in the crusher.

The installation of FIG. 1 proposes a reserve 22 of inert gas (e.g., a set of nitrogen bottles or of C0,), connected with pipe 11 by pipe 23, on which a shutter 24 is interposed; this'shutter is controlled by an oxygen detector 25 mounted on pipe II. In cases where for any reason the gas circulating in this pipe would contain an oxygen amount above 17 percent, the detecting device would open shutter 24, and the inert gas of source 22 would spread in the circulating system and eliminate the dangerous oxygen.

The installation of FIG. 2 differs from the one of FIG. 1 insofar as there is a temperature exchanger 26 interposed on pipe 9; here the calories still present in the gas circulating in this pipe are transferred to surrounding air taken in 27, to be discharged into pipe 28. In the presented example, the air thus heated is conveyed through pipes 29 and 30, both equipped with shutters 31 and 32, into enclosures 33 and 34, representing, for instance, tunnels intended for the various treatments of the boards, such as preheating and conditioning. No details are provided for the construction of these enclosures, which could be of any appropriate type, connected or disconnected. It will be noted, however, that while enclosure 33 is in direct contact, i.e., the air coming from its inlet pipe 29 circulates directly around the treated products 35, the air in enclosure 34 heats only the exterior walls of an inner tunnel, containing the products 36 to be treated, and it is equipped with its own circulating device. 1

Another difference between the installations of FIG. 1 and FIG. 2 consists of the fact that from the discharge of pipe 28 a part of the heated air of exchanger 26 is taken and conducted through pipe 37 to fan 38, to feed the burners l5. Naturally, this additional heating of the combustive air permits to reduce the fuel quantity, while maintaining unchanged conditions in the combustion chamber. On the other hand, it is easy to realize that through the intervention of the temperature exchanger 26, the gases recuperated by pipe 11 are colder than in the case of FIG. 1.

Pipe 26 a indicates the drain for condensed water recovered from the gases circulating in pipe 9, due to their cooling.

It is clear that by comparison with FIG. 1 the installation of FIG. 2 has the advantage that the gas recovered by pipe 11 carries a distinctly lower quantity of water vapor; this fact increases the drying potential of the secondary mixture circulating in pipe 18. Moreover, a recuperation of calories is achieved which might be of considerable interest in certain cases, since it results in a decrease of fuel consumption per ton of finished product.

In the variant of FIG. 3 heat recovery and moisture condensation are effected exclusively for the gas introduced into recirculating pipe 11, on which, for this purpose, a chamber 39 is interposed; here cold water'is vaporized which arrives from pipe 40. One could, however, also effect the recovery from all of the gas discharged by the drier. This water is received by a pump 41 and conveyed into an air heater 42, associated with a chamber or tunnel 43. A draining pump for the used water is indicated by 44. A part of the hot water delivery of pump 41 passes through pipe 45 which opens into an exchanger 46, representing a device for a preheating combustive air for burners 15. It is a matter of course that in this case again the air carried by pipe 11 into pipe 18 will be markedly colder, just as in the equipment of FIG. 2.

Naturally, the cooling and condensing devices of FIGS. 2 and 3 are perfectly interchangeable, both with regard to the kind of apparatus and-their disposition within the installation.

In what has been described above, the circulation system conveying the oxygen-poor gas mixture concerns only the crusher and the drier of the machinery for making fiber boards. It does not affect the conveyance system carrying dry fibers to the forming wire apparatus nor the apparatus itself.

Of course, the latter could also be included in the circulations of FIGS. 1, 2, and 3, but in practice this would entail complications better to be avoided. Therefore, it is preferable to equip the fonning apparatus with a second circulating system, more or less independent of the first one.

The example of FIG. 4 shows an apparatus 47 discharging 12 fibers, suitably connected with the outlet of separator 5 of FIGS. l-3. Apparatus 47 discharges dry fibers into a hopper 48, which conveys them to the suction of a fan 49 through an air-lock 49a isolating it from the surrounding air. From here the fibers are carried pneumatically through pipe 50, opening into difiuser S1 of a distribution box 52. The latter covers a forming wire 53 made of wire gauze, which circulates along the perforated upper surface 54 of a suction box 55. The fibers settle on the wire carpet 53 to constitute the mattress 56, while the conveyance gas penetrates into box 55; the latter is connected by pipe 55a with the suction of fan 57. The delivery of this fan enters pipe 58 leading to a cyclone separator 59, which is equipped with an air-lock 59a. Fibers having passed through perforated surface 54 are separated in this cyclone and reenter the feeding apparatus 47, whereas the gas escapes through pipe 60, to again reach the suction of fan 49.

Of course, if the circulation system just described were perfectly tight, one could till it with an inert gas or with a mixture poor in oxygen, to be certain that phenomena of ignition or explosion could never occur. But in fact the pipe system has numerous gas leaks and infiltration points of atmospheric air, so that it is necessary to supply it continually or discontinually with an additional suitable gas mixture.

In the represented example such an addition is provided by pipe 18 of FIG. 1 to 3 by means of an intake 61, equipped with a regulating shutter 62. Pipe 61 passes through an exchanger 63, where the gas mixture transfers calories to water coming in through pipe 64 and leaving, heated, through pipe 65; this hot water could be used, for instance, to enter a broiler producing the steam required for boiling the wood chips before they are crushed. The cooled gas mixture is conducted by'pipe 66 into pipe 60 described above, i.e., to the suction of fan 49. It will be noted that in the represented example pipe 61 is branched up with pipe 18 above the junction point of the latter with. recirculating pipe 11; this for the purpose of feeding the circulation with an oxygen-poor gas, namely approximately 14.3 percent in the given example, thus taking eventual air reentries into account.

A gas detector 67 for the oxygen proportion is inserted in the circulation system (on pipe 50 in the example); this detector controls a regulating shutter 68 on pipe 61. By this arrangement the supplementary gas mixture is supplied to the circulating system according to its needs. As a variant, detector 67 can also command a shutter 69, mounted on pipe 70, which connects a source of inert gas 71 with pipe 66, i.e., with fan 49. If a considerable discharge can be had from source 71, the latter could supply the needed addition instead of pipe 61, which then would normally be kept closed. But as shown, it is also possible to use the two arrangements simultaneously (i.e., shutters 68 and 69). One would then proceed by having detector 67 control only shutter 68 in normal conditions, but in cases where, shutter 68 being entirely open, the oxygen proportion would continue to rise, detector 67 would open shutter, 69, in order to introduce a sufficient quantity of inert gas into the pipe system and eliminate any risk of inflammation or explosion.

As can be seen on FIG. 4, a roller 72, pressing onto mattress 56, is placed at the exit opening of distribution box or forming chamber 52. Detail diagram FIG. 5 shows more clearly that this roller flattens the mattress as it leaves the forming chamber in order to assure a relative airtightness between the distribution box and the atmosphere; this does not result in any disadvantage, as the mattress in question will be subjected anyway to the action of compressing cylinders before reaching the heating plate press. To assure airtightness between roller 72 and the walls of the box, a flap 73, bearing against the roller and sealing the space between its outer surface and the wall, is attached at the upper edge of the opening provided in the box for placing the roller. Moreover, as shown on FIG. 7, the bearing blocks 74 supporting shaft 75 of the roller are slidably mounted in guides 76 (FIG. 7), integral with the walls; the blocks 74 can be adjusted by means of threaded stems 77 (FIGS. 6 and 8) and nuts 78. Lids 79 (FIG. 6) cover these mechanisms to ensure tightness. If wanted, shaft 75 can be made projecting through a flexible wall 80, to carry a suitable driving pinion 81.

Cover flaps 82 are attached between the walls of box 52 and the forming wire mat 53, at the sides and towards the entrance of the wire gauze, in order to limit air reentry from without or gas leaks from within.

Iclaim:

l. A process for preparing fibers which are pneumatically conveyed and then pressed into fiberboard, the improvement comprising eliminating fire and explosion hazards, by:

a. forming a substantially closed, controlled atmosphere by mixing heated combustion gases with ambient air to cool said gases and introducing said cooled gases, as said atmosphere, to a crusher wherein wood chips are formed into fibers,

b. pneumatically conveying said fibers by said cooled gases from said crusher to a drier,

c. separating the dried fibers from said cooled gases,

d. delivering said dried fibers to a receiving hopper,

e. recirculating at least a portion of said separated gases with said heated combustion gases introduced to said crusher, f. regulating the percentages of mixed combustion gases,

ambient air and recirculated gases to maintain the controlled atmosphere introduced to said crusher at less than 17 percent oxygen, at less than [00 grams of water vapor per kilogram of dry gases, and at a temperature of less than 350 C.

g. pneumatically conveying, with the controlled atmosphere, the fibers from said hopper, and collecting said fibers upon a wire gauze through which the gases of said atmosphere pass,

h. separating any fibers remaining in the gases passing through said gauze, and

i. recycling said separated remaining fibers and gases into said hopper and said controlled atmosphere respectively.

2. The process of claim 1, comprising monitoring the oxygen content of said controlled atmosphere and adding an inert gas to said atmosphere should the oxygen content exceed 17 percent.

3. The process of claim 1 wherein said combustion gases are further cooled within a heat exchanger with a fluid.

IF =0 1F 

2. The process of claim 1, comprising monitoring the oxygen content of said controlled atmosphere and adding an inert gas to said atmosphere should the oxygen content exceed 17 percent.
 3. The process of claim 1 wherein said combustion gases are further cooled within a heat exchanger with a fluid. 